Do you mean only after the router reboot, then the router can detect your HDD? If you don't restart the router, is the USB light on after you plug in the HDD? What's the size of your HDD?

The HDD is a WD 4TB NTFS formatted drive. When I plug it in, the green light at the back of the router goes on but when I enter the menu go to USB storage and click on scan, the drive is not visible. When I make a restart the drive is shown and works fine, but for me it is not a good solution tho make a restart every time I plug in the drive.

What's the firmware version of your C7 V2? You may upgrade the firmware if it is not the latest. And also try another HDD or USB drive to see whether it will do the same.